1. Field of the Invention
This invention relates to electrically enchanced production of liquid hydrocarbons from slowly producing subsurface formations through a borehole extending from the surface of the earth to the formation. More specifically, this invention relates to the optimized disposition of electrodes surrounding a borehole for energy efficient heating of the formation to maximize production of the liquid hydrocarbons from portions of the formation substantially beyond the electrodes while minimizing cost.
2. Description of the Prior Art
In many deposits, especially in medium and heavy oil deposits, tar sand deposits, and light oil deposits containing paraffins, the viscosity of the oil impedes flow, especially in the immediate vicinity of the borehole through which the oil is being produced. As all of the oil must flow into the borehole, the mobility of the fluid in the immediate vicinity of the borehole dominates the production rate. Any impediment to fluid flow at the borehole is particularly unwelcome.
It is known to heat the oil formations, particularly in the vicinity of the borehole, to lower the viscosity of the liquids in the deposit and hence provide greater mobility and more profitable production. Steam injection has been used to heat a deposit to reduce the viscosity of the oil in the vicinity of the borehole. To some extent steam can be used as a heat transport medium and steam can be used on some deposits economically. However, if steam is injected from the surface it loses a large amount of heat as it progresses down the hole, wastefully heating formations above the formation of interest. This has given impetus to the development of downhole steam generators, which, in turn, have problems of their own. Further, this use of steam stimulation is uneconomic in many deposits.
A number of electric heating methods have been considered for formations in which water is present, as it is in most formations, in the intersticial spaces in a low-loss medium, such as quartz sandstone. It is known to provide uniform heating of such a formation by inter-well energization, as shown, for example, in Bridges and Taflove, U.S. Reissue Pat. No. Re. 30,738. Such methods require relatively extensive boreholes and comprehensive development of the field, which is not always warranted. Others, for instance Kern, U.S. Pat. No. 3,848,671, have proposed use of multiple electrodes to heat almost all of the deposit non-uniformly as a preconditioning step prior to a fluid replacement process. Some methods are directed to deposits which do not flow without stimulation. Specific target formations for this approach are oil shale and tar sand deposits which lack native drive. Here, heating must be excessive because of the high temperature needed to convert the solid-like hydrocarbonaceous material to free-flowing fluids. Single well heating is shown in Sarapuu, U.S. Pat. No. 3,211,220, which shows the application of electric power between an electrode in a formation and a distributed electrode at or near the earth's surface.
It has been proposed that single well stimulation is more effective if heat can be applied some distance into the formations from a borehole. To this end it has been suggested to extend the electrodes themselves from the borehole laterally out into the formation. See, for example, Kern U.S. Pat. No. 3,874,450; Todd U.S. Pat. No. 4,084,639; Gill U.S. Pat. No. 3,547,193; Crowson U.S. Pat. No. 3,620,300; and Orkiszewski, et al. U.S. Pat. No. 3,149,672. In Crowson U.S. Pat. No. 3,620,300 there is shown a method and system wherein not only the electrodes but also insulating barriers are extended out into the formations.
A method of borehole enlargement using lateral drain holes which can also be practiced in combination with electric heating is described by Perkins (U.S. Pat. No. 4,489,782). Perkins' method involves completing a production well with lateral drain holes extending from the borehole in the formation, which drain holes produce in conjunction with electric stimulation arising from using the drain holes as electrodes. The use of lateral drain hole schemes can raise additional questions associated with regulatory restrictions upon the number of producing wells per acre. This invention operates under the constraint of enhancing production of liquid hydrocarbons through traditional boreholes with traditional production borehole spacing.
Bridges, et al. have described single well stimulation methods using either a single applicator or a set of two electrodes disposed inside the borehole (U.S. Pat. No. 4,524,827). The methods described by Bridges, et al. produce highly concentrated heating patterns around the borehole.
Gill, U.S. Pat. No. 3,642,066, as an augmentation to his electro-osmosis treatment, teaches also heating a formation through passage of current from a borehole to an electrode well. Gill does not teach surrounding a borehole with an integrated array of electrodes or ring-like electrodes. Gill does not teach passing current between the electrodes to the exclusion of the borehole surrounded. Gill does not teach the necessity of optimizing the dimensions and configurations of the array together with the power expended in relation to formation geometry and geophysics to achieve a synergistic effect.
It is a feature of the present invention to enhance the recovery of liquid hydrocarbons from a slowly producing subsurface formation through a borehole extending from the surface of the earth into the formation in an improved manner wherein only a limited portion of the formation is heated by the application of optimum electric power between an optimally configured interrelated electrode array disposed in the formation around the borehole, or between such electrode array and a return electrode disposed near the earth's surface, the effect being that the viscosity of the liquid hydrocarbons near the producing borehole is reduced, the pressure gradient is redistributed further out in the formation and the enhanced production is net energy effective.
It is another feature of the present invention to enhance the recovery of liquid hydrocarbons from a slowly producing subsurface formation through a borehole extending from the surface of the earth into the formation in an improved manner wherein only a limited portion of the formation is heated by the application of electric power between ring-like electrodes disposed in the formation around the borehole, or between such an electrode and a return electrode disposed near the earth's surface, the effect being that the viscosity of the liquid hydrocarbons near the producing borehole is reduced, the pressure gradient is redistributed further out in the formation and the enhanced production is net energy effective.
It is another feature of the present invention to provide for such enhanced recovery of liquid hydrocarbons from a slowly producing subsurface formation by electrically heating the formation through a ring-like electrode implanted in the formation around the borehole in an improved manner wherein the ring-like electrode is approximated by a plurality of electrode segments.
It is another feature of the present invention to provide for such enhanced recovery of liquid hydrocarbons from a slowly producing subsurface formation by electrically heating the formation through electrodes disposed in the formation around the borehole in an improved manner wherein one of the electrodes is a segment of electrically conductive borehole casing.
It is a feature of the present invention to provide such enhanced recovery in an improved manner through a traditional producing borehole in the formation under the constraint of traditional production well spacing.